Automatic sprinkler heads with temperature-responsive fusible elements have long been used in all types of buildings for fire protection. There is a need, however, for a sprinkler especially suited for residential use. In the past, residential sprinklers were of the type originally developed for factories and like applications. To serve in residential applications, a sprinkler valve should have a faster (e.g., a factor of four) response time than has been needed in many conventional applications, i.e., it must turn on sooner at a given air temperature. The faster response is needed to detect the presence of a fire soon enough to prevent buildup of noxious gases (e.g., carbon monoxide) from burning materials (e.g., plastics) commonly found in residences. The faster response is also desired to better contain and extinguish flames.
Government and industry organizations have conducted research on the required speed of response, which indicates that a 21 second or smaller time constant is necessary. The time constant is measured as the period of time required for the fusible element in the sprinkler to achieve 63% of the rise in temperature occasioned by inserting the sprinkler from ambient into a stream of heated air flowing through a duct. For example, were the sprinkler moved from equilibrium at 70.degree. F. into a stream of 360.degree. F. air, the time constant would be the amount of time required for the fusible element to reach 63% of the total rise in temperature, or 63% of 290.degree. F. The time constant is not, of course, necessarily the amount of time required to turn on the sprinkler in a fire, but rather an indication of the relative speed of response.
To obtain a fast response, prior sprinkler heads (e.g., Russell U.S. Pat. No. 1,834,319, Loepsinger U.S. Pat. No. 1,919,235, and Russell U.S. Pat. No. 1,932,805) have employed sheet-metal linkage members in contact with a fusible medium to reduce the thermal mass to be heated, and have added fins and flanges to the linkage members to conduct heat toward the fusible element. Russell U.S. Pat. No. 1,932,805 also teaches that flanges extending along the curved edges of one of the sheet-metal pieces can direct rising currents of warm air against the soldered surfaces between the flanges. Other efforts in the field are Teague U.S. Pat. No. 1,300,046, Loepsinger et al. U.S. Pat. No. 2,075,816, and Gloeckler U.S. Pat. No. 3,874,456.